This application is based upon, claims the benefit of priority of, and incorporates by reference Japanese Patent Application No. 2003-109109 filed Apr. 14, 2003.
1. Field of the Invention
The present invention relates to a system for calculating an air-fuel ratio of each cylinder of a multicylinder internal combustion engine, and more particularly to a technique whereby, using an air-fuel ratio sensor installed in an exhaust collecting portion of a multicylinder internal combustion engine, an air-fuel ratio of each cylinder is suitably calculated based on a sensor detection signal from the air-fuel ratio sensor.
2. Description of the Related Art
Generally, an air-fuel ratio control system detects an exhaust gas-fuel ratio of an internal combustion engine and controls the fuel injection quantity to achieve a target air-fuel ratio. However, in a case where this type of control system is used for a multicylinder internal combustion engine, the intake air quantity will vary among the cylinders depending on intake manifold configurations, intake valve operation, or the like. Also, in case of an MPI (multi-point injection) system whereby cylinders are each provided with a fuel injection valve so that fuel is injected individually to each cylinder, the injected fuel quantity will vary among the cylinders due to inherent differences of fuel injectors. These variations among the cylinders are detrimental to the accuracy of controlling the fuel injection quantity. An example of countermeasures to this problem is disclosed in Japanese Patent Laid-Open Publication No. Hei 8-338285 (1996), according to which, when an air-fuel ratio is detected by an air-fuel ratio sensor, a particular cylinder corresponding to the exhaust for which the air-fuel ratio is to be actually detected is identified, so that the air-fuel ratio is individually feedback controlled for each identified cylinder each time.
Additionally, according to Japanese Patent Publication No. Hei 3-37020 (1991), the air-fuel ratio at an exhaust collecting portion is detected by the use of an air-fuel ratio sensor, and the fuel quantity supplied to the corresponding cylinder is corrected by taking into consideration the time delay caused by the exhaust from the corresponding cylinder to arrive at the air-fuel ratio sensor.
However, with the technologies as disclosed above, since exhaust gases from the various cylinders are mixed together at the exhaust collecting portion, the variations among the cylinders will not be eliminated sufficiently, and thus there is a desire for further improvement with respect to this aspect. The technology disclosed in Japanese Patent Publication No. Hei 3-37020 (1991), in particular, is only effective for the case where the exhaust gas forms layers in the direction of the pipes. It is true that the air-fuel ratio for each cylinder can be obtained with a high degree of accuracy by arranging an air-fuel ratio sensor at each branched pipe of the exhaust manifold; however, this requires the same number of air-fuel ratio sensors as the number of the cylinders, resulting in a cost increase.
The publication of Japanese Patent No. 2717744 discloses a technology whereby the air-fuel ratio at an exhaust collecting portion is modeled as a mixture of air-fuel ratios of a plurality of cylinders and an air-fuel ratio of each cylinder is detected by means of an observer in terms of the internal state as the air-fuel ratio of each cylinder. However, since the air-fuel ratio at the exhaust collecting portion is an air-fuel ratio of the mixture of exhaust gas from the cylinders, it is not possible to accurately detect the air-fuel ratio of each individual cylinder if the exhaust gas quantity is changed or if there is variation in exhaust gas quantity among the cylinders.
In view of the drawbacks as stated above, an object of the present invention is to provide a system that is capable of calculating an air-fuel ratio of each cylinder of a multicylinder internal combustion engine with a high degree of accuracy, and thus capable of improving the accuracy of fuel injection control performed using the individual cylinder air-fuel ratio.
In a multicylinder internal combustion engine according to an embodiment of present invention, a plurality of exhaust passages connected to respective cylinders are merged together at an exhaust collecting portion and an air-fuel ratio sensor is arranged at this exhaust collecting portion. The air-fuel ratio sensor detects an air-fuel ratio in the state where exhaust gases from the cylinders are mixed together. In this regard, accurate calculation of the air-fuel ratio of each of the cylinders (individual cylinder air-fuel ratio) is required for controlling the fuel injection quantity for each cylinder accurately without any excess or deficiency. Since it is believed that an air-fuel ratio at the exhaust collecting portion obtained from a sensor detection signal from the air-fuel ratio sensor is affected by the mixture of exhaust gases in the exhaust system, the present invention proposes a technique that enables accurate calculation of an individual cylinder air-fuel ratio by giving due consideration to the effects due to the mixture of exhaust gases. Specifically, such consideration is given to the effects due to the mixture of exhaust gases by reflecting the gas flow rate history of each of the cylinders.
More specifically, according to an aspect of the invention, the air-fuel ratio at the exhaust collecting portion is calculated by using a sensor detection signal from an air-fuel ratio sensor while at the same time the gas flow rate at the exhaust collecting portion is calculated based on the gas flow rate history of each cylinder. Further, on the basis of the air-fuel ratio at the exhaust collecting portion and the gas flow rate at the exhaust collecting portion thus obtained, the fuel quantity at the exhaust collecting portion is calculated as the burnt fuel quantity corresponding to these values. Furthermore, an observer using an individual cylinder fuel quantity as a variable is constructed by a model in which the collecting portion fuel quantity is associated with the individual cylinder fuel quantity, so that the individual cylinder fuel quantity is estimated from the result of observation by the observer. The individual cylinder air-fuel ratio is then calculated using the individual cylinder fuel quantity thus estimated. In a fuel injection control system, the fuel injection quantity for each of the cylinders is feedback controlled by using the individual cylinder air-fuel ratio obtained each time. An observer computes a variable, which is defined by a model.
According to the constitution described above, the gas flow rate at the exhaust collecting portion is calculated based on the gas flow rate history of each cylinder, so that the thus obtained gas flow rate at the exhaust collecting portion duly reflects variations among the cylinders. Accordingly, it is possible to calculate the fuel quantity at the collecting portion, duly reflecting the effects of the mixture of exhaust gases in the exhaust circuit, and hence to improve the accuracy of the individual cylinder fuel quantity estimated by the observer. As the result, an accurate individual cylinder air-fuel ratio can be obtained even if the intake air quantity differs from cylinder to cylinder. In this manner, it is possible to calculate the individual cylinder air-fuel ratio accurately and hence to improve the accuracy of fuel injection control that is performed using the individual cylinder air-fuel ratios. Note that the individual cylinder gas flow rate includes the individual cylinder intake air quantity introduced into each of the cylinders as well as the exhaust gas quantity discharged from each of the cylinders.
The model in which the collecting portion fuel quantity is associated with the individual cylinder fuel quantity may be, according to another aspect of the invention, a model in which the collecting portion fuel quantity is associated as a weighted average of a fuel quantity history of each cylinder.
According to yet another aspect of the invention, the individual cylinder fuel quantity is estimated by a Kalman filter type observer. The Kalman filter type observer handles an estimated value statistically so that errors can be absorbed. Therefore, it is possible to more accurately obtain the individual cylinder air-fuel ratio.
According to still another aspect of the invention, the gas flow rate at the exhaust collecting portion is calculated by using a model in which the gas flow rate for each cylinder is associated with the gas flow rate at the exhaust collecting portion. Thus, it is possible to calculate the gas flow rate at the exhaust collecting portion, duly reflecting cylinder-to-cylinder variations in gas flow rate (exhaust gas quantity or intake air quantity).
According to yet another aspect of the invention, in the system for calculating the air-fuel ratio of the immediately prior aspect, the gas flow rate at the exhaust collecting portion may be calculated by using a model in which the weighted average of the gas flow rate history of each cylinder is associated as the gas flow rate at the exhaust collecting portion.
According to still another aspect of the invention, the system further utilizes a filter means for compensating the phase delay of the sensor detection signal sent by the air-fuel ratio sensor so that the collecting portion fuel quantity is calculated based on the output from the filter means and the gas flow rate at the exhaust collecting portion. Specifically, the air-fuel ratio sensor sends output (sensor detection signal) with a time delay after receiving an input (i.e. actual air-fuel ratio), and therefore the waveform of the output is rounded in response to the change of input. If the phase delay of the sensor detection signal is compensated by the filter means, a true input without time delay can be obtained and hence the accuracy of the individual cylinder air-fuel ratio can be improved. Variations due to inherent differences or changes with time can also be absorbed. For example, the filter means may be constituted by a Kalman filter so that the actual air-fuel ratio can be correctly estimated.
Further, according to another aspect of the invention that is based on the premise that as exhaust gases from the cylinders are merged at the exhaust collecting portion, the air-fuel ratio at the exhaust collecting portion is detected by an air-fuel ratio sensor, and an individual cylinder air-fuel ratio is calculated based on a sensor detection signal from the air-fuel ratio sensor. Particularly, the air-fuel ratio at the exhaust collecting portion is calculated based on the sensor detection signal and the gas flow rate history of each cylinder. Further, an observer using the individual cylinder air-fuel ratio as a variable is constructed by a model in which the air-fuel ratio at the exhaust collecting portion is associated with the individual cylinder air-fuel ratio so that the individual cylinder air-fuel ratio is estimated based on the result of observation by the observer. In the fuel injection control system constituted in this manner, the fuel injection quantity for each of the cylinders is feedback controlled using the individual cylinder air-fuel ratio obtained each time.
According to the constitution of the immediately preceding aspect, since the air-fuel ratio at the exhaust collecting portion is calculated reflecting the gas flow rate history of each cylinder, the air-fuel ratio at the exhaust collecting portion can be obtained while giving due consideration to variations in the gas flow rate among the cylinders. Therefore, the accuracy of the individual cylinder air-fuel ratio estimated by the observer can be improved and the calculation error can be minimized even if each cylinder has a different intake air quantity. As the result, it is possible to calculate an individual cylinder air-fuel ratio with high precision and thus to improve the accuracy of fuel injection control performed using this individual cylinder air-fuel ratio.
According to another aspect of the invention, the air-fuel ratio at the exhaust collecting portion is calculated by using a sensor model for compensating the phase delay of a sensor detection signal output by the air-fuel ratio sensor. Specifically, the air-fuel ratio sensor sends an output (sensor detection signal) with a time delay after receiving an input (i.e. actual air-fuel ratio), and therefore the waveform of the output is rounded in response to the change of input. If the phase delay of the sensor detection signal is compensated by the sensor model, a true input without time delay can be obtained and hence the accuracy of the individual cylinder air-fuel ratio can be improved. Variations due to inherent differences or changes with time can also be absorbed. For example, the sensor model may be constituted by a Kalman filter so that the actual air-fuel ratio can be correctly estimated.
According to another aspect of the invention, in case of a multicylinder internal combustion engine having an intake control system capable of controlling the intake air quantity for each of the cylinders, the intake air quantity tends to vary among the cylinders. However, according to the invention, an air-fuel ratio of each individual cylinder can be calculated accurately even in such a case. An example of the known intake control systems is one employing an adjustable valve mechanism constructed such that the conditions for opening/closing the intake valve (e.g. valve lift quantity and opening/closing timing) can be variably adjusted on a continuous basis.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.